A silicon photomultiplier (hereinafter also referred to as SiPM) is a photodetection element in which avalanche photodiode cells (hereinafter referred to as APD cells) are two-dimensionally arranged in parallel. As the APD cells are activated at a higher reverse bias voltage than the breakdown voltage of the APD cells, the SiPM is driven in a region called Geiger mode. The gain of each APD cell in a Geiger-mode operation is 105 to 106, which is extremely high, and thus, very weak light from a single photon can be measured.
A high resistor called a quench resistor is connected in series to each APD cell. When a photon enters and causes Geiger discharge, the amplification effect ends due to a voltage drop caused by the quench resistor. As a result, a pulse output signal is obtained. In a SiPM, each APD cell functions in this manner. Therefore, when Geiger discharge is caused in more than one APD cell, an output signal with a charge amount or a pulse height value several times greater than that of the APD cells that have caused the Geiger discharge is obtained as an output signal from one APD cell. Accordingly, the number of the APD cells that have caused the Geiger discharge, or the number of photons that have entered the SiPM, can be measured from an output signal. Thus, photon measurement can be conducted for each one photon.
The device characteristics of an SiPM include photon detection efficiency, which serves as a sensitivity indicator, gain, the dark count rate of noise components due to a thermal factor, crosstalk of noise components due to light emission occurring in an avalanche process, and afterpulse of noise components due to carrier capture and re-emission. All of these characteristics depend on overvoltage (defined as “drive voltage-breakdown voltage”).
As described above, the device characteristics of an SiPM have high dependence on overvoltage, and greatly vary with drive conditions. For example, an SiPM can be driven at a low overvoltage in a system expected to be operated in low-noise environments, and can be driven at a high overvoltage in a system with its priority put on sensitivity. The critical aspect here is the uniformity in breakdown voltage among the APD cells constituting the SiPM. An output signal of the SiPM is formed by superimposing output signals of the respective APD cells connected in parallel. Therefore, output signals are handled on the assumption that the APD cells have uniform characteristics in principle. However, if the breakdown voltages vary, the characteristics of the APD cells vary, and the charge amounts of output signals from the APD cells also vary. As a result, the photon measurement accuracy becomes lower. Therefore, to conduct high-accuracy photon measurement, it is necessary to provide an SiPM without variation in breakdown voltage.
An SiPM having a vertical structure is known. In this SiPM, a PN junction is formed at an interface between a semiconductor region (P-type) formed through epitaxial growth and a semiconductor region (N-type) of a substrate, and a depletion layer spreads on the light receiving surface side as a reverse bias voltage is applied. When the depletion layer reaches the edge of the high-doped semiconductor region of the light receiving surface, and continues to sweep the reverse bias voltage, avalanche breakdown occurs. That is, the breakdown voltage is determined by the thickness of the epitaxial layer or the depth of the semiconductor region of the light receiving surface formed by ion implantation. The acceptable range of variation in the thickness of the epitaxial layer in a wafer plane due to epitaxial growth is normally approximately ±2% of the thickness. Therefore, if the thickness of the epitaxial layer is designed to be 3 μm, the variation is approximately 120 nm.
Meanwhile, the breakdown voltage of an SiPM is attributed to the APD cell structure, but varies in a range of 20 mV/nm with thickness variation. Therefore, where the thickness variation is 120 nm, the variation in breakdown voltage is approximately 2.4 V at a maximum, and the device characteristics of the APD cells in a Geiger mode will vary greatly. However, controlling the thickness of the epitaxial layer to several nanometers so as to reduce the variation in breakdown voltage might lead to increases in technical difficulties and costs.